1. Field of Invention
This invention relates to a method of firing of cores made of low density graphite/alumina mixtures and in particular to a method which includes the use of apparatus for enhancing the firing of the cores.
2. Description of the Prior Art
Aluminum oxide cores produced by a process embodying a material mixture of alumina and a reactant fugitive filler mixture requires special treatment during firing to prevent distortion of the cores. The cores are referred to as low density graphite/alumina (LDGA) type cores. The processing of these LDGA type cores requires that close attention must be given to the possible chemical reactions which can occur in the aluminum-oxygen-carbon system during firing of the cores. It has been determined that the initial reaction which takes place during firing of the core is expressed by the following formula: EQU 2Al.sub.2 O.sub.3(s) +3C.sub.(s) .fwdarw.Al.sub.4 O.sub.4 C.sub.(s) +2CO.sub.(g)
Once the above reaction has taken place, the partial pressure of carbon monoxide, P.sub.CO, begins to decrease and the Al.sub.4 O.sub.4 C phase disappears by a chemical reaction which is not understood at this time. The disappearance of the Al.sub.4 O.sub.4 C phase can only occur if the P.sub.CO is .ltoreq.0.1 atmosphere. It is for this reason that it is important that the flow of the ambient furnace gas be as uniform as possible about the core during firing.
To date, microstructural examination of fired core cross-sections have revealed more pronounced density gradients on core surfaces exposed to the furnace atmosphere than on core surfaces in contact with the core setter. The examination and conclusions drawn therefrom indicates that the Al.sub.2 O gas formed by decomposition of Al.sub.4 O.sub.4 C was condensing to form Al.sub.2 O.sub.3 preferentially on the exposed core surfaces. Consequently, the cores became distorted during the firing process.
It is therefore an object of this invention to provide a new and improved method of firing low density graphite/alumina core compacts to achieve a good gas flow about and through the porous structure of the compact.
Another object of this invention to provide new and improved method which includes the use of a novel setter design for supporting low density graphite/alumina cores and enhancing reaction product removal during firing.
Another object of this invention is to provide new and improved method for firing low density graphite/alumina cores wherein good turbulent gas flow is achieved over substantially all of the surface areas of the core.
Other objects of this invention will, in part, be obvious and will, in part, appear hereinafter.